1. Field of the Invention
The present invention relates to a lithium nickel copper composite oxide and its production process and use. Especially, said composite oxide has the use as a positive electrode active material of a nonaqueous secondary battery which can be charged and discharged with a great capacity and has an excellent cycle characteristic.
2. Related Art
Recently, a secondary battery is often used as an electric power source of a portable electronic device in terms of economy. There are many kinds of secondary batteries. Among them, a nickel-cadmium battery is most popular now and recently a nickel-hydrogen battery is becoming popular. Meanwhile, a lithium secondary battery using lithium for an electrode produces high output voltage and has high-energy density. Therefore, the lithium secondary battery is practically utilized in some extent, and is now eagerly studied to improve its performance.
One of the materials for a positive electrode of the lithium secondary battery, which is commercially available now, is LiCoO.sub.2. However, cobalt as the raw material of LiCoO.sub.2 is expensive. Therefore, it is recognized that LiNiO.sub.2 will be a good material for the positive electrode of the lithium secondary battery for next generation, because LiNiO.sub.2 has the same crystallographic structure and shows almost the same electrochemical behaviors as LiCoO.sub.2, and nickel as the raw material of LiNiO.sub.2 is less expensive than cobalt.
It is known that LiNiO.sub.2 is more stable than any other lithium nickel composite oxide and can be prepared relatively with ease. In the case where LiNiO.sub.2 is used for the positive electrode active material of the nonaqueous secondary battery, lithium atoms are electrochemically deserted from the crystallographic structure as a result of charging.
Nickel exists in the state of Ni.sup.3+ in the crystal of LiNiO.sub.2. When lithium atoms are deserted from crystal of LiNiO.sub.2, nickel is oxidized from 3+ into 4+. Maximum oxidation number of nickel is 4+ and Ni.sup.4+ cannot be oxidized any more. Even if all lithium atoms are electrochemically drawn out from the crystal of LiNiO.sub.2, the electrical capacity of the positive electrode never becomes more than one electron equivalent. Namely, the electrical capacity of the positive electrode is theoretically no more than 274.6 mAh/g.
However, when the lithium secondary battery using LiNiO.sub.2 as the positive electrode active material is actually charged and discharged, the capacity of the positive electrode is around half of the above-mentioned theoretical capacity. Electrochemically, the capacity of the positive electrode can be up to about 70% to 80% of the above-mentioned theoretical capacity. However, in this case, the battery is so much deteriorated on its cycle characteristic and can be no more in practical use.
LiNiO.sub.2 has so-called layer structure made from Li layers and Ni--O layers. When too much lithium atoms are electrochemically deserted from Li layers, the repulsion between Ni--O layers grows larger and destroys the crystallographic structure of LiNiO.sub.2 itself. This is the reason for the above deterioration. In the case where lithium atoms are electrochemically deserted from the Li layers, nickel atoms from the Ni--O layers also fall into lithium sites and this causes the deterioration of the battery capacity.
There are many kinds of ways to overcome these disadvantages. However, the above phenomena come from the basic structure of LiNiO.sub.2, so that it is impossible to find the fundamental way for overcoming these disadvantages. For the above-mentioned reasons, there are fundamental problems if the known lithium nickel composite oxide such as LiNiO.sub.2 is used as the positive electrode active material of the secondary battery. Therefore, there has been a longing for a new positive electrode active material in order to provide the lithium secondary battery having excellent properties, especially large charge/discharge capacity.
As a negative electrode active material, on the other hand, usable are metallic lithium, lithium alloys (e.g., lithium aluminum or the like), carbon materials, substances which can be intercalated and deintercalated with lithium ions (e.g., conductive polymers such as polyacetylene, polythiophene, poly-p-phenylene or the like), transition metal oxides, transition metal sulfide, transition metal nitride, lithium transition metal oxide, lithium transition metal sulfide or lithium transition metal nitride. These materials may be used alone or as a composite thereof.
Among them, when metallic lithium or lithium alloy such as lithium aluminum is used as the negative electrode active material, the battery thereof has the great capacity per unit weight and high energy density. However, in this case, dendritical crystal, i.e., so-called dendrite appears and grows on the surface of the metallic lithium during the repeated charging and discharging processes. In a short time, the grown dendrite may contact the positive electrode to cause a short circuit within the battery or, in the worst case, a danger of firing.
Therefore, it is preferred that as the negative electrode active material for secondary battery used is the material which is not metallic lithium or lithium aluminum alloy and can be intercalated and deintercalated with lithium ions into the inside. Among them, it is thought that the carbon material is the most promising material in terms of energy density and price.